This invention relates, in general, to photo sensors (also called photo detectors) and is directed specifically to an improvement in drive and sensing circuitry for photo sensors.
This invention can best be understood by a reference to FIG. 1 of the drawings in which a schematic of a prior art photo sensor drive and sensing circuitry is disclosed.
In FIG. 1, it can be seen that the components of the photo sensor 10, ie, a light emitting diode (LED) 12 and a phototransistor 14 are connected by two conductor wires W.sub.1 and W.sub.2 in the conventional manner. One wire is connected between a positive voltage supply +Vcc and the LED and the other wire is connected between the phototransistor 14 and a sensing (driven or output) circuitry 16, shown as a PNP transistor 20 connected to a Schmitt trigger 22.
When a number of optical sensors of this prior art type are used, wiring and design becomes somewhat complex. Each optical sensor circuit requires different components since each circuit has to be tailored (adjusted) for each LED due to a wide spread in acceptable parameters for each LED. Also, since the current through each LED is a fixed value and each LED is biased fully ON in such prior art circuits with 15 to 20 milliamperes of current (15 to 20 milliamps.times.the number of LEDs used), there is considerable current being used. Also, this current is being used whether or not the phototransistor is detecting the presence (light blockage) or the absence (no light blockage) of an object between it and the LED.
It is an object of this invention to reduce the wiring complexity where a number of optical sensors are used and to eliminate the need for selecting different components, ie, to eliminate the need to tailor each circuit for each LED. That is to say that when the same brand photosensor-LED unit is used in a number of places in an electrical mechanical device, it is not necessary to use different bias resistors for each circuit. In accordance with this invention all circuits will have the same LED bias network designed for the worst case.
Still another object of this invention is to reduce the current through each photo sensor circuit in stand-by mode thus reducing the overall current required when a number of photo sensors are used.
Still another object of this invention is to simplify the circuitry and to increase the reliability of photo sensors, mainly because of the reduced LED current in stand-by mode.